Fast-switching, high-capacity, optical switches are needed to realize an agile optical-core network. That is, an optical-core network that may adjust swiftly to changes in desired connectivity between edge nodes. Switching latency in a given optical node may preclude the use of the given optical node for time-sharing schemes such as Time Division Multiplexing (TDM) switching or burst switching. In the absence of such time-sharing schemes, the given optical node becomes a channel-switching cross-connector and a network based on such an optical node may be forced to perform multiple edge-to-edge hops to inter-connect particular edge nodes. This performing of multiple hops can significantly increase the complexity of, and degrade the performance of, a network.
In a channel-switching scheme, an optical switch may be arranged to switch an input channel to an output channel. The input channel may be defined by an input link in which the input channel is received as well as the wavelength that is modulated to carry the information transmitted on the input channel. Similarly, the output channel may be defined by an output link in which the output channel is transmitted as well as the wavelength that is modulated to carry the information transmitted on the output channel.
In a TDM-switching scheme, the information arriving on a given input channel may be destined for a number of different output channels. A TDM frame of a predetermined duration is defined to be divided into a number of equal duration time slots. An edge node continually sends TDM frames to an optical switch arranged to perform TDM-switching. In each time slot of an input channel may be information destined for a different output channel. The optical switch must be arranged to switch the input channel to the appropriate output channel for each time slot. Furthermore, a mechanism must be in place by which the optical switch can anticipate exactly when to expect the beginning of each TDM frame to arrive from an edge node.
McGuire (U.S. Pat. No. 5,889,600, issued Mar. 30, 1999) discloses a modular switch operated in a channel switching mode comprising a plurality of star couplers, connecting to a plurality of Input Wavelength Division Multiplexed (WDM) links and a plurality of output WDM links. Each WDM link comprises a number of wavelength channels equal to the number of star couplers. Each input WDM link is demultiplexed into the constituent wavelength channels and each of the constituent wavelength channels connects to an input port of one of the star couplers. Wavelength converters are provided at the output ports of the star couplers. Each output WDM link carries an optical signal that is made up of wavelength channels received from an output port of each star coupler multiplexed together. The modular switch allows a wavelength channel from any input port to connect to any wavelength channel in a subset of the output ports of the star couplers. For example, using 32×32 star couplers, 32 WDM input links and 32 WDM output links, each input link and each output link carrying 32 wavelength channels, a specific wavelength channel in an input link can be switched to any one of a subset of 32 output ports of the 1,024 output ports of the 32 star couplers.
Multi-stage, optical switch structures that switch channels are known. For example, Kuroyanagi (U.S. Pat. No. 6,154,583, issued Nov. 28, 2000) describes an optical switch configured as a multi-stage circuit, with each of the stages including a plurality of space switches. An arrangement of optical amplifiers is also described. Such structures, however, are limited to channel switching granularity, which may be considered too coarse for future applications.
Bala et al. (U.S. Pat. No. 6,335,992, issued Jan. 1, 2002) describe a scalable, multi-stage, optical cross-connect. The multi-stage optical cross connect comprises a plurality of first stage switch matrices, a plurality of middle stage switch matrices, and a plurality of last stage switch matrices. Each of the first stage switch matrices has a number of input ports, each input port receiving an input communication signal, and a larger number of output ports, where the first stage switch matrices switch the input communication signals to selected output ports. The input ports of the middle stage switch matrices are coupled to the output ports of the first stage switch matrices for receiving communication signals output from the first stage switch matrices. The middle stage switch matrices switch communications signals received at their input ports to their output ports. The input ports of the last stage switch matrices are coupled to the output ports of the middle stage switch matrices for receiving communication signals output from the middle stage switch matrices. The last stage switch matrices switch communications signals received at their input ports to their output ports. In addition, the middle stage itself can be recursively a multistage switch.
Neither of the above approaches suggests the use of a time-sharing scheme, such as TDM, in a bufferless modular switching node. A node structure that permits scalability and can (employ time-sharing techniques is required, and methods of circumventing the difficulty of scheduling signal transfer are needed to enable the realization of such nodes and, ultimately, the realization of an efficient agile network that scales to capacities of the order of several petabits/second.